Voltage sensitive control device

ABSTRACT

A power supply monitor circuit which disconnects a load from the alternating current supply lines when the output voltage from the supply drops below a predetermined level for an interval of time exceeding a certain predetermined tolerable interval. The output voltage level of the supply is sensed by a voltage divider connected in parallel with the supply, and a Zener diode is used to establish a lower threshold at which a silicon controlled rectifier (SCR) can be fired. The SCR is connected in series with a load cut-out relay to thereby control the relay energization. A time delay circuit is associated with the gate electrode of the SCR to allow conduction thereof for a short time period following a drop in supply voltage, thus establishing the tolerable interval and permitting and accommodating momentary voltage drops.

VOLTAGE SENSITIVE'CONTIRQL DEVICE Inventor: Irvin I. Lee, Minneapolis,Minn.

Assignees Lear Siegler, Inc., Minneapolis,

Minn.

Filed: May 30, 1972 Appl. No.: 257,509

References Cited UNITED STATES PATENTS Harmon et al....

Elpers Zocholl 317/36 TD 3,566,198 2/1971 Delahunty.. 317/33 SC3,611,050 10/1971 Weber 317/31 FOREIGN PATENTS OR APPLICATIONS 317/33 SC317/33 SC Oct. 30, 1973 Primary Examiner-Gerald Goldberg Attorney-L.Paul Burd et al.

[57] ABSTRACT A power supply monitor circuit which disconnects a loadfrom the alternating current supply lines when the output voltage fromthe supply drops below a predetermined level for an interval of timeexceeding a certain predetermined tolerable interval. The output voltagelevel of the supply is sensed by a voltage divider connected in parallelwith the supply, and a Zener diode is used to establish a lowerthreshold at which a silicon controlled rectifier (SCR) can be fired.The SCR is connected in series with a load cut-out relay to therebycontrol the relay energization. A time delay circuit is associated withthe gate electrode of the SCR to allow conduction thereof for a shorttime period following a drop in supply voltage, thus establishing thetolerable interval and permitting and accommodating momentary voltagedrops.

2 Claims, 2 Drawing Figures 1,360,312 3/1964 France ..,.....317/33 SC I0([12 I4 usv 22 MONITORED CIRCUIT I 44 34 30 5 I /38 CR "32 ll mlPMENIEDBBI 30 I975 SHEET 2 BF 2 VwN m0 QDN mmw H536 awmotzoz I VOLTAGESENSITIVE CONTROL DEVICE BACKGROUND OF THE INVENTION This inventionrelates generally to a brown-out protection circuit of the type employedto protect sensitive load elements, such as alternating current motors,from damage due to a partial loss or drop in the voltage being suppliedthereto, and more specifically to such a circuit which is constructedfrom solid-state components and is arranged to yield fail-safeoperation.

In Gilbert U.S. Pat. No. 3,389,325, there is described a system of thetype herein described which is responsive to a drop in supply voltage todisconnect a motor from the supply mains. In the Gilbert circuit, thedrop in supply voltage causes a Zener diode to cut off to thereby fire auni-junction transistor, which, in turn, causes a silicon controlledrectifier to fire. This energizes a relay to cut the load out of thesupply circuit. In the event of a total loss of power, however, thecontrol relay will not be energized and cannot serve to protect the loaduntil power is restored.

SUMMARY OF INVENTION In the preferred embodiment of the presentinvention, as soon as the supply voltage drops below a prescribedthreshold for a predetermined time interval,

such as below a level of 90 percent of the nominal or rated voltage. Thecontrol relay is de-energized and the associated normally open contactsdisconnect the load from the power supply. This occurring even whenthere is a total loss of power. This operation is accomplished byconnecting the control relay in a series circuit with a SCR and inparallel with the load to be protected. The normally open contacts ofthe relay are disposed between the alternating current supply and theload, and remain closed only so long as the relay winding is energized.The conduction of the SCR on alternate halfcycles of the AC supply iscontrolledby a time delay network and a threshold setting network suchthat the SCR is able to be fired for only a short predetermined periodfollowing a steady or continuous drop of potential below the threshold.This arrangement permits short-termed fluctuations in the AC supply andonly serves to separate the load from the supply when the drop persistsfor a given period.

It is accordingly the object of this invention to provide a novel,fail-safe, low cost, reliable line voltage monitoring and disconnectcircuit for protecting a load from the effects of a drop in supplyvoltage.

This and other objects of the invention will become apparent from areading of the accompanying detailed description, especially in light ofthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates one embodiment ofthe invention in which a manual reset is used to re-initiate operationfollowing the resumption of full power; and

FIG. 2 illustrates a modification of the circuit of FIG. 1,incorporating means for automatically reinitiating proper operationfollowing a loss of power.

DESCRIPTION OF THE FIG. I EMBODIMENT Referring to FIG. 1, there is shownone embodiment of a brown-out protection circuit. The terminals and 12are adapted to be connected to a source of alternating current voltageto be monitored. A pair of lines l4 and 16 are respectively connected tothe terminals 10 and 12. Connected in series between junction points 18and 20 in line 14 is a normally open relay contact 22. Connected inparallel with contact 22 is a normally open, manually operablepush-button switch 24. The load 25 to be protected is adapted to beconnected between the junction point 20 and a junction 26 on line 16.Also disposed in line 14 is a fuse link 28 which is included to provideprotection to the various components in the brown-out protector circuit.

Connected across the lines 14 and 16 between junction points 30 and 32is a series circuit including a relay 34 and a silicon controlledrectifier (SCR) 36. The relay 34, when energized with sufficientcurrent, causes the contacts 22 to be held closed. The vSCR 36 has acathode electrode 40, an anode electrode 38 and a gate electrode 42. Theanode electrode 38 is. connected at junction 44 to one terminal of therelay 34. The cathode electrode is connected by a conductor 46 to thejunction point 30 on line 14. The gate electrode 42 is coupled through aresistor 48 to a junction point 50. Also coupled to the junction point50 is a first terminal of a parallel combination of a resistor 52 and acapacitor 54. The other terminal of this parallel combination isidentified by numeral 56 on line 14. A diode 57 is disposedin the line16 and serves to half-wave rectify the applied alternating current inputapplied at terminals 10 and 12.

Connected in series across the lines 14 and 16 are a pair'of resistors,58 and 60 which act and function as a voltage divider. In the embodimentof FIG. 1, resistor 58 should be in the range of, for example, fromthree to ID times the ohmic value of resistor 60, with this range beingsuitable for a variety of purposes, and with other ranges being useful.A Zener diode 62 connects the junction point 50 to ajunction 64 which isthe common terminal between resistors 58 and 60. A capacitor 66 isconnected across the lines 14 and 16 between the junction points 68 and70 and is therefore in parallel circuit with the voltage dividerresistors 58 and 60. This capacitor serves as a filter to smooth out theripples in the half-wave rectified direct current developed across thevoltage divider comprised of resistors 58 and 60 and to suppresstransient signals which might otherwise adversely affect the operationof the circuit components. Finally, a capacitor 72 is connected inparallel with the relay winding 34 across the junction points 32 and 44.

Now that the details of the construction of the embodiment of FIG. 1have been explained, consideration will be given to the mode ofoperation of the brown-out protector circuit.

OPERATION FIG. 1

In operation, the terminals 10 and l2'are connected to asource ofalternated current potential such as a ll5-volt, 60-cycle supply, suchthat a half-wave rectified direct current potential will be developedacross the resistors 58 and 60. With this connection established andwith the magnitude of the voltage divider resistors 58 and 60 properlyselected, when the pushbutton switch 24 is momentarily closed, gatecurrent will be provided through the Zener diode 62 and the resistor 48to the gate electrode 42 of the silicon controlled rectifier 36. Hence,for the half-cycle of the alternating current supply, when the terminal12 is positive with respect to the terminal 10, the SCR 36 will be firedinto its low impedance state and sufficient current will flow throughthe relay coil 34 to actuate the contacts 22 to'their closed position sothat the manual switch 24 can be released. At the same time, a chargewill be built up on the capacitor 72 so that when the po- 5 larity ofthe AC supply is reversed on the subsequent half-cycle to therebyextinguish the SCR 36, the residual charge on capacitor 72 will maintainthe relay 34 energized during this half-cycle so that the contacts 22will remain closed. 1 Y

The Zener diode 62 provides a threshold for the brown-out protectioncircuit so that it will operate to disconnect the load from the ACsupply lines when the supply voltage falls below this threshold or levelfor an interval greater than a predetermined period of time.Specifically, by properly setting the ratio of resistor 58 to resistor60, and by including the Zener diode 62 in series with the resistor 48between the gate terminal 42 and the junction point 64, when the voltageacross the lines 14 and 16 drops to a predetermined level, say 90percent of its nominal or normal rated value, insufficient gate currentwill be supplied to the SCR 36 to allow it to be tired. l-lence,'thecharge on capacitor 72 will decay and will not be restored on alternatehalfcycles and the relay coil 34 will be de-energized. The de-energization of relay 34 will permit the contacts 22 to open and therebydisconnect the load 25 from the alternating current supply.

To insure that short term reductions in supply voltage below the desiredthreshold will not cause the load to be disconnected, a time delaycircuit includingthe capacitor 54 and the'resistors 48 and 52 isprovided. The charge which is stored in capacitor 54 by the currentwhich flows through the Zener diode 62 when biased to its conductingstate .is sufficient to maintain the flow of gate current throughthe SCR36 for a predetermined number of cycles of the AC supply even though thevoltage of the supply has dropped below the established threshold andthe Zener diode is substantially non-conducting. It is only after thetime taken by the charge of capacitor 54 to leak off or decay throughthe resistor 52 and 48 that the SCR 36 will be prevented from firing onits appropriate half-cycle. By proper selection of the values ofresistor'52 and capacitor 54, this time delay can be made arbitrarilylong.

Following a drop in supply voltage in excess of this predeterminedperiod, it is again necessary to manually re-establish the connection ofthe load and the monitoring circuit to ,the AC supply by closing thepushbutton switch 24 as previously described. Provided the supplyvoltage is above the established threshold determined by the voltagedivider resistors 58 and 60 and the Zener diode 62, sufficient gatecurrent will flow Type c 1068 SCR Resistor 48 100,000 ohms Resistor 52100,000 ohms Resistor 58 68,000 ohms Resistor 60 22,000 ohms- Capacitor72 5 microfarads 10 microfarads 5 microfarads 100 Volt, 1 Watt Type1N2069 Capacitor 54 Capacitor 66 Zener Diode 62 Diode 57 In FIG. 2 thereis illustrated another embodiment of a brown-out protection circuitwhich provides for the automatic re-initiation of the load circuit whenfull power is restored following a brown-out condition.

Referring to FIG. 2, the circuit components added to the embodiment ofFIG. 1 to attain automatic reset are shown enclosed by the dashed lineboxes 200 and 202. The circuit components, conductors, and junctionpoints which are the same as those illustrated in FIG. 1, are given thesame identifying numeral except that these numerals are preceded by thenumber 2 to clearly indicate that they are associated with theembodiment of FIG. 2.

The terminals 210 and 212 are adapted to be connected to a source ofalternating current voltage to be monitored. A pair oflines 214 and 216are respectively connected to the terminals 210 and 212. A conductor 246connects the cathode electrode 240 of SCR 236 to a junction point 230 onconductor 214. The gate electrode of SCR 236 is connected by way of aresistor 248 to a junction point 250. The parallel combination of asecond resistor 252 and a capacitor 254 is connected between thejunction 250 and the junction 256 on line 214. A Zener diode 262connects the junction point 250 to the wiper terminal of a voltagedivider potentiometer 258 which is connected between a junction point268 on conductor 214 and a junction point 270. Connected directly inparallel with the voltage divider potentiometer 258 is a capacitor 266.The cathode electrode of a conventional semiconductor diode 257 isconnected to the junction point 270. The anode electrode of diode 257 isconnected through a resistor 211 to a junction point 213 to whichis'alsoconnected' a first terminal of a pair of resistors 215 and 217and the anode electrode of a Zener diode 219. The other termi-' nal ofresistor 215 is connected to the collector electrode of a transistor221. The other terminal of resistor 217 is connected to a junction point223 to which is connected the emitter electrode of the transistor 221.While the'transistor 221 is illustratedas being of the PNP type, it isobvious to those skilled in the art that a NPN transistor type can besubstituted provided proper polarity conventions are observed. Thejunction 223 is connected by a conductor 225 to the junction point 232.A resistor 227 connects the base electrode of transistor 221 to theconductor 225. The winding of a relay 234 is connected between thejunction point 232 and a junction 229 on a conductor 231. A resistor 233is connected. between junction point 229 and the base electrode of thetransistor 221.

Associated with the relay winding 234 is a set of double pole-doublethrow relay contacts 235. The load to be protected (not shown) isadapted to be connected in circuit with these contacts so as to bedisconnected from its associated power supply when the voltage appliedthereto drops below a predetermined threshold for a pre-establishedperiod of time. Disposed between the junction 229 on conductor 231 andthe anode electrode 238 of the SCR 236 is the circuitry contained withinthe dashed box 200. More specifically, the conductor 231 is connected toa junction 237 which has connected thereto a first terminal of aresistor 239, a

first terminal of a capacitor 241 and the anode electrode of a secondsilicon controlled rectifier (SCR) 243. The second terminal of theresistor 239 is connected by a conductor 245 to a pair of junctionpoints 247 and 249. The other terminal of the capacitor 241 is connectedto a junction point 251 on a conductor 253. The cathode electrode of theSCR 243 is connected to junction 255 to which is also connected theanode electrode of SCR 236.

Connected between the junction point 249 and a junction 259 on conductor253 is a voltage divider including a resistor 261 and a capacitor 264.The junction between the resistor 261 and capacitor 264 is connected tothe emitter electrode of a uni-junction transistor (UJT) 265. A firstbase terminal of UJT 265 is connected to the junction point 247 onconductor 245. The remaining base terminal of UJT 265 is connected to ajunction point 267 to which is also connected a first terminal of acapacitor 269 and a resistor 271. The other terminal of capacitor 269 isconnected to the gate electrode of SCR v243 while the other terminal ofthe resistor 271 is connected to a junction point on conductor 253. Aresistor 273 also connects the gate electrode of SCR 243 to a junctionon the conductor 253. A Zener diode 275 has its anode electrodeconnected to the junction point 259 on conductor 253 and its cathodeelectrode connected to the junction point 249 on conductor 245 and istherefore directly in parallel with the voltage divider comprised ofresistor 261 and capacitor 264. Finally, the capacitor 272 is connectedbetween a junction on conductor 216 and the junction point 251 onconductor 253.

Now that the details of the construction and layout of the embodiment ofFIG. 2 have been described, consideration will be given to its mode ofoperation.

OPERATION FIG. 2

Let it be initially assumed that the terminals 210 and 212 have beenconnected to a suitable source of alternating current which has beenmaintained at rated value for some period of time. Under these-assumedconditions, the SCRs 236 and 243 will have gate current applied theretoand therefore will be rendered conductive to provide half-waverectification. The relay winding 234 will accordingly be energized andthe capacitor 272 will have its charge periodically replenished and actsas a filter for the half-wave rectified DC current supplied to the relaycoil 234. Under the previously assumed conditions, the UJT 265 and thetransistor 221 will both be conducting.

If it is now assumed that the supply voltage drops for a time exceedinga predetermined interval determined by the RC time constant establishedby resistor 252 and capacitor 254 such that capacitor 254 can no longersupply the requisite gate current for SCR 236, SCR 236 will be turnedoff, thus disconnecting capacitor 272 from the supply which is connectedto the terminals 210 and 212. After the charge stored on capacitor 272is dissipated, the relaycoil 234 will be de-en'ergized and the contacts235 associated therewithwill switch so as to disconnect the load (notshown) from the source of supply voltage. The turning off of SCR 236also serves to disconnect the SCR 243 from the supply and it too isrendered non-conductive. Uni-junction transistor 265 and transistor 221will also be rendered nonconductive following the turning off of SCR236.

if it is next assumed that the brown-out condition ends and the powerreturns to its normal rated value, the current flowing through resistor217, resistor 211, diode 257 and the parallel combination of resistor258 and capacitor 266 will be half-wave rectified by the diode 257. Asthe voltage builds up across the voltage divider 258, the thresholdestablished by the Zener diode 262 will be exceeded and gate currentwill again flow through resistor 248 and the Zener diode 262 such thatSCR 236 will fire at the point in the cycle when terminal 212 ispositive with respect to terminal 210.

The firing of SCR 236 serves to connect the series combination of therelay coil 234 and the capacitor 241 across the supply lines 214 and216, with practically the entire supply voltage being dropped across thecapacitor 241. This serves to suppress transient spikes initiated by thefiring of SCR 236 and prevents false firing of the SCR 243. The voltageon capacitor 241 is divided by the voltage divider comprised of resistor239 and the Zener diode 275. This combination of resistor and Zenerdiode fix the voltage across the series circuit including resistor 261and capacitor 264 at the nominal value of the Zener diode. These lastmentioned components comprise a time delay circuit in that as the chargebuilds up on capacitor 264 by way of resistor 261, a point is reached atwhich the UJT 265 will be rendered conductive. Once UJT 265 isconducting, capacitor 264 discharges and a positive pulse is coupledthrough the capacitor 269 to the gate electrode of SCR 243 causing it tofire.

With both SCR 236 and SCR 243 conducting, the relay winding 234 will beenergized with sufficient current to cause its associated contacts toreverse their position. The firing of the SCR 243 also alters the biasapplied to the transistor 221 by way of resistor 233 and transistor 221is rendered fully conducting. Because resistor 215 is small incomparison to resistor 217, the effect of turning on transistor 221 isto shunt out the resistor 217 such that an increased voltage isdeveloped across the capacitor 266. This action provides a predetermineddifferential between the firing level and extinction level of the SCR236. Thus it will only be when the power again drops below a pre-setthreshold that the SCR 236 will be rendered non-conductive to againdisconnect the load from the source.

The timing of the firing of the uni-junction transistor 265 by theresistor 261 and capacitor 264 time delay circuit insures that thealternating current supply is up to its maximum level before the load isconnected to the supply by the energization of the relay winding 234.

The function of the Zener diode 219 is to limit the voltage developedacross resistor 217 and therefore prevents the charge on capacitor 266from exceeding the voltage rating of the transistor 221 which mightotherwise burn it out. The resistor 211 acts as a current limiter forthe Zener diode 219.

' component types and values which may be used in constructing thecircuit of FIG. 2 for general use on 115- volt, 60-cycle supply systems.

Capacitors 272, 266 Capacitors 254. 264 Capacitors 241, 269

5 microfarad/25O V. microfarad/SO V. 0.] microfarad/200 V.

Thus, it can be seen that this invention is effective to provideprotection to a load circuit of the type which might otherwise bedamaged by an insufficient supply voltage.

I claim:

l. A power supply monitoring circuit for disconnecting a load devicefrom a power supply in the event of a sustained drop in supply voltage,said circuit including:

a. first and second lines adapted to be connected to a source ofalternating current;

b. a relay having a coil and a set of normally open contacts associatedtherewith;

c. a first triggerable switching means having a pair of outputelectrodes and a control electrode;

, d. a second triggerable switching means having a pair of outputelectrodes and a control electrode;

e. means connecting said relay coil and the output electrodes of saidfirst and second triggerable switching means in series between saidfirst and second lines;

f. a resistive voltage divider connected in parallel with the seriescombination of said first and second triggerable switching means andsaid relay coil;

g. means including a voltage threshold establishing device connectingsaid voltage divider to the control electrode of said first triggerableswitching means, such that said relay coil is maintained energized andsaid contacts are maintained closed so long as the voltage applied tosaid first control electrode exceeds said threshold;

h. delay means connected to said first control electrode for maintaininga triggering signal thereon for at least a first predetermined timeperiod following said drop in supply voltage, said delay means connectedto said first control electrode including a resistor and a capacitorconnected in parallel with each other and in series with said voltagethreshold establishing device; f.

i. timing means associated with theelectrode of said second triggerableswitching means, said timingmeans being operative upon triggering ofsaidfirst triggerable switching means to apply triggering signals to saidsecond control electrode a predetermined time following the triggeringof said first triggerable switching means. 5 2. A power supplymonitoring circuit for disconnecting a load device from a power supplyin the event of a sustained drop in supply voltage, said circuitincluding:

a. first and second lines adapted to be connected to a source ofalternating current;

b. a relay having a coil and a set of normally open contacts associatedtherewith;

c. a first triggerable switching means having pair of output electrodesand a control electrode;

d.,a second triggerable switching means having a pair of outputelectrodes and a control electrode;

e. means connecting said relay coil and the output electrodes of saidfirst and second triggerable switching means in series between saidfirst and second lines;

f. a resistive voltage divider connected in parallel with the seriescombination of said first and second triggerable switching means andsaid relay coil;

g. means including a voltage threshold establishing device connectingsaid voltage divider to the control electrode of said first triggerableswitching means, such that said relay coil is maintained energized andsaid contacts are maintained closed so long as the voltage applied tosaid first control electrode exceeds said threshold;

h. delay means connected to said first control electrode for maintaininga triggering signal thereon for at least a first predetermined timeperiod following said drop in supply voltage, said delay means connectedto said first control electrode including a resistor and a capacitorconnected in parallel with each other and in series with said voltagethreshold establishing device; timing means associated with the controlelectrode ofvsaid second triggerable switching means, said timing meansbeing operative upon triggering of said first triggerable switchingmeans to apply triggering signals to said second control electrode apredetermined time following the triggering of said first triggerableswitching means;

j. said timing means including:

i. a resistor and a capacitor connected in series between one of saidoutput terminals of said first triggerable switching means and saidrelay coil, and I 2. a uni-junction transistor having a pair of outputelectrodes and a control electrode, the control electrode of saiduni-junction transistor being connected to the common point between saidresistor and capacitor, and the pair of output electrodes of saiduni-junction said transistor being coupled'to the control electrode ofsaid second triggerable switching means.

1. A power supply monitoring circuit for disconnecting a load devicefrom a power supply in the event of a sustained drop in supply voltage,said circuit including: a. first and second lines adapted to beconnected to a source of alternating current; b. a relay having a coiland a set of normally open contacts associated therewith; c. a firsttriggerable switching means having a pair of output electrodes and acontrol electrode; d. a second triggerable switching means having a pairof ouTput electrodes and a control electrode; e. means connecting saidrelay coil and the output electrodes of said first and secondtriggerable switching means in series between said first and secondlines; f. a resistive voltage divider connected in parallel with theseries combination of said first and second triggerable switching meansand said relay coil; g. means including a voltage threshold establishingdevice connecting said voltage divider to the control electrode of saidfirst triggerable switching means, such that said relay coil ismaintained energized and said contacts are maintained closed so long asthe voltage applied to said first control electrode exceeds saidthreshold; h. delay means connected to said first control electrode formaintaining a triggering signal thereon for at least a firstpredetermined time period following said drop in supply voltage, saiddelay means connected to said first control electrode including aresistor and a capacitor connected in parallel with each other and inseries with said voltage threshold establishing device; i. timing meansassociated with the electrode of said second triggerable switchingmeans, said timingmeans being operative upon triggering of said firsttriggerable switching means to apply triggering signals to said secondcontrol electrode a predetermined time following the triggering of saidfirst triggerable switching means.
 2. a uni-junction transistor having apair of output electrodes and a control electrode, the control electrodeof said uni-junction transistor being connected to the common pointbetween said resistor and capacitor, and the pair of output electrodesof said uni-junction said transistor being coupled to the controlelectrode of said second triggerable switching means.
 2. A power supplymonitoring circuit for disconnecting a load device from a power supplyin the event of a sustained drop in supply voltage, said circuitincluding: a. first and second lines adapted to be connected to a sourceof alternating current; b. a relay having a coil and a set of normallyopen contacts associated therewith; c. a first triggerable switchingmeans having pair of output electrodes and a control electrode; d. asecond triggerable switching means having a pair of output electrodesand a control electrode; e. means connecting said relay coil and theoutput electrodes of said first and second triggerable switching meansin series between said first and second lines; f. a resistive voltagedivider connected in parallel with the series combination of said firstand second triggerable switching means and said relay coil; g. meansincluding a voltage threshold establishing device connecting saidvoltage divider to the control electrode of said first triggerableswitching means, such that said relay coil is maintained energized andsaid contacts are maintained closed so long as the voltage applied tosaid first control electrode exceeds said threshold; h. delay meansconnected to said first control electrode for maintaining a triggeringsignal thereon for at least a first predetermined time period followingsaid drop in supply voltage, said delay means connected to said firstcontrol electrode including a resistor and a capacitor connected inparallel with each other and in series with said voltage thresholdestablishing device; i. timing means associated with the controlelectrode of said second triggerable switching means, said timing meansbeing operative upon triggering of said first triggerable switchingmeans to apply triggering signals to said second control electrode apredetermined time following the triggering of said first triggerableswitching means; j. said timing means including: